Gastrointestinal (GI) mucosa is one of the most radiosensitive tissues and radiation-associated GI injury could be fatal. We have developed a small molecule (Rx100) with potent radioprotecting/radiomitigating efficacy in GI tract. In response to Request For Application (RFA-AI-07-013) "Medical Countermeasures to Restore Gastrointestinal Function after Radiation Exposure: Project Bioshield (RC1)" issued by NIAID, we submit this application focusing on preclinical development of Rx100 as a radioprotector/radiomitigator against radiation-associated GI damage. We propose four specific aims with the ultimate goal to provide the U.S. government and mankind with a product effective in protecting life from radiation injury. 1. Optimize the formulation and delivery route of Rx100 to provide maximum efficacy in mice. Rx100 will be formulated using different carriers, and an optimal formulation will be chosen for further evaluation. 2. Explore synergisms between Rx100 and three existing radioprotectors/radiomitigators: alphadifluoromethylornithine (DFMO), amifostine (WR-2721) and 5-androstenediol (5-AED). 3. Evaluate the capacity of optimal Rx100 regimens, either alone or in combinations, to restore the GI barrier function after radiation exposure: Glucose absorption, mucosal permeability and bacterial invasion. 4. Acute toxicity study and pharmacokinetic profile of Rx100 Methods: (1) Microcolony Assay measures the capacity of GI stem cells to survive radiation injury. Mice are sacrificed 3.5 days after whole-body exposure to a Cs137 source. (2) Bone-Marrow-Shielding Model to select GI responses reflects mouse survival attributed to drugs'GI protection. (3) Analysis of recovery of villus structure up to 10 days postirradiation reflects the crypt regeneration dynamics. (4) GI glucose absorption and permeability will be measured in mice using radio-labeled tracers. Bacterial translocation will be evaluated by appropriate microbiological methods. (5) LD50 of Rx100 will be determined in acute toxicity study. (6) PK study using high-performance liquid chromatography mass spectrometry (LC/MS/MS). Relevance to public health: Exposure to radiation could result in fatal damage to the gut. No satisfactory radioprotectors/radiomitigators are available yet. We propose to develop Rx100 into a highly efficient medical countermeasure against radiological and nuclear threats, and a potent drug for cancer patients.